6 Symptoms of Bad Spark Plug On a Motorcycle

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A spark plug is a critical ignition system component for any internal combustion engine. It ignites the compressed air-fuel mixture at the end of the compression stroke, which pushes the piston to the bottom dead center (BDC).

If a spark plug fails to ignite the air-fuel mixture, your motorcycle will not start. Moreover, there are several symptoms that your motorcycle will show if the spark plug goes bad.

So, what are the signs and symptoms of a bad spark plug on a motorcycle? The common symptoms of a bad spark plug are engine misfiring, exhaust backfiring, poor acceleration, strong-smelling smog from the exhaust, a flooded engine, and difficulty starting. You can also physically inspect the spark plug for burns, cracks, rusted or corroded tips, and carbon fouling.

Motorcycle Dirty Spark Plug

Generally, a copper spark plug lasts over 20,000 miles, while platinum and iridium plugs last over 60,000 and 100,000 miles, respectively. However, replacing the spark plug between 15,000 to 20,000 miles is recommended for better engine performance.

6 Signs of a Bad Spark Plug on Motorcycles

If you’ve owned a motorcycle for several years, chances are you’ve experienced the frustration of spark plug problems, especially if you ride an older bike like I do.

I have restored many old motorcycles and replaced over a hundred spark plugs because older ones require them. All of them show some common signs when the spark plug goes bad.

I have explained the six common symptoms you’ll experience when your motorcycle spark plug goes bad.

1. Engine Misfire

The most common symptom of a bad spark plug is an engine misfire. If you notice your motorcycle engine goes off for a fraction of a second and get back to normal, it’s a sign of misfiring.

It causes because the spark plug fails to completely ignite the air-fuel mixture in the combustion chamber, resulting in a loss of power.

This can be quite alarming and significantly impact your riding experience. So, if your motorcycle is misfiring, physically inspect the spark plug and replace it if it’s gone bad.

2. Exhaust Backfire

It could be a sign of a faulty spark plug if you’re experiencing sudden loud pops or bangs from your exhaust while decelerating or during gear changes.

This phenomenon is popularly known as exhaust backfiring. It occurs due to incomplete combustion of hydrocarbons in the combustion chamber.

Once unburnt hydrocarbons come in contact with air, they ignite and create a loud pop or gunfire sound.

If your spark plug fails to deliver a strong spark to ignite all molecules of the air-fuel mixture, a backfire occurs.

The too-rich or lean air-fuel mixture is also a reason for exhaust backfiring. So, you have to tune your carburetor and replace the spark plug to fix backfiring.

3. Flooded Engine

When the spark plug fails to ignite the air-fuel mixture effectively, an excessive amount of fuel can accumulate in the combustion chamber.

This fuel accumulation leads to a flooded engine. This makes starting the motorcycle challenging, particularly during cold starts or after several unsuccessful attempts to start the bike.

To fix a flooded engine, remove the spark plug, turn off the fuel supply, and kickstart several times. The piston will push all excess fuel from the combustion chamber.

After several attempts with a closed fuel supply, install the spark plug, open the fuel supply, and start the motorcycle.

4. Reduced Mileage

If you notice a significant decrease in fuel efficiency or reduced mileage, it’s worth checking the condition of your spark plugs.

A bad spark plug can lead to incomplete combustion, wasting fuel and impacting your motorcycle’s overall fuel economy.

There are several more reasons for lower MPG, but a bad spark plug is the most common reason for reduced fuel efficiency.

5. Difficult Starting

A worn-out or damaged spark plug can make starting your motorcycle a real challenge. You may experience prolonged cranking, multiple attempts, or even complete failure to start. This can be frustrating, especially when you’re eager to hit the road.

6. Poor Acceleration

A deteriorating spark plug can hinder your motorcycle’s acceleration. You may notice sluggish or delayed responses when you twist the throttle, making achieving the desired speed and performance harder.

If your motorcycle engine hesitates to throttle response and seems like losing power while accelerating, it’s a sign of a bad spark plug.

You must immediately replace the spark plug to improve your riding experience. Click here to read my other guide on how to fix poor acceleration and throttle response.

What Causes Spark Plug To Go Bad

Now, you know the various symptoms of a bad spark plug on motorcycles. Don’t ignore your spark plug if you notice any of the above symptoms.

But what causes the spark plug to go bad? Your spark plug bears extremely high temperature and pressure. So, over time, the nobs on the end of the spark plug simply wear out from sparking.

Here are some common causes of bad spark plugs:

  • Bad timing
  • Lean air-fuel mixture
  • Faulty piston rings
  • Age

How Often You Should Replace Motorcycle Spark Plug

Your motorcycle’s spark plug continuously bears frequent changes in pressure and temperature.

So, the nobs on the end of the spark plug simply wear out from sparking, and it goes bad.

That’s why you must replace the motorcycle’s spark plug between 15,000 to 20,000 miles or every five years.

Spark plugs are inexpensive and very easy to replace. You don’t need to go to any professional mechanic shop for spark plug replacements.

How To Replace a Motorcycle Spark Plug

Replacing a motorcycle spark plug is a very easy process. It needs a new spark plug, some tools, and 10 to 15 minutes.

To replace the motorcycle spark plug, follow these steps:

  1. Put your motorcycle on the center stand.
  2. Switch off the ignition key.
  3. Remove the old spark plug with a special tool in the motorcycle’s toolbox.
  4. Clean the cylinder head with cotton or microfiber cloth.
  5. Thread in the new spark plug (same thread size and heat range) and tighten it fully.

That’s it! You’ve replaced your motorcycle spark plug within 10 minutes. Now, connect the ignition wire to the spark plug and start the engine.

Note: Don’t forget to tighten the spark plug fully; otherwise, you’ll face pressure leakage, which reduces your engine horsepower.


The motorcycle’s spark plug should be replaced between 15,000 to 20,000 miles or every five years. It goes bad or loses spark generation capacity over time, which reduces fuel efficiency. If you notice the above symptoms while riding, replace the spark plug.

Let me know if you’re facing any difficulties related to your motorcycle engine. Use the comment box to ask your queries.

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